Router OSPF is the command used to enable OSPF routing on a Cisco router. But before we get into the command syntax I want to give you a brief summary of some important OSPF details for the ICND2 and CCNA Exam.
Dynamic Routing Protocol
OSPF is a Dynamic Routing Protocol that can be used across many different brands of routers and not just Cisco. For a little refresher, a dynamic routing protocol is able to adapt quickly to networking changes in case a link breaks or a router ceases to function. It will be able to find an alternate route automatically so your whole network doesn’t come crashing down as apposed to static routes where the network administrator would have to update all the new routes manually.
Interior Gateway Protocol
There are interior gateway protocols and exterior gateway protocols. An interior gateway protocol works with a single group of networks and an exterior gateway protocol connects different groups of networks together. OSPF is an interior gateway protocol.
Interior gateway protocols have three different types of ways to determine the best route between routers. There are distance vector, link-state, and a mixture of the two. OSPF uses link-state.
Many different routing protocols can be configured on a Cisco router at a time. In order for the routers to determine which routing protocol should take priority they use administrative distance.
The lower the administrative distance the better. A directly connected route has an AD of 0 and a statically configure route has an AD of 1. OSPF has an AD of 110. As a comparison to other routing protocols EIGRP has an AD of 90 and RIP has an AD of 120. This means that if you have OSPF and RIP configured in a network, the routers will choose the routes created by OSPF over RIP.
Link-State
The main difference between link-state and distance vector routing protocols is that link state will only send updates when there is a topology change. Distance vector however send updates at regular intervals, such as every 30 seconds. As a comparison OSPF sends its topology table out every 30 minutes. Which is quite a bit longer time to wait to here about updates. However, OSPF makes up for its infrequent topology exchanges by using link-state advertisements which are tiny updates that get sent out immediately after a change is detected.
So, the benefit of link-state is that it can use less networking resources with fewer updates and can converge much faster. Convergence means that all the routers in the network have finished discussing the changes that have occurred and are no longer sending updates back and forth.
Shortest Path First
OSPF uses what is called the SPF algorithm to calculate the shortest paths between routers. Even though each OSPF router has the exact same topology, they each will have a different point of view because the SPF algorithm gets calculated on each router individually.
Areas
OSPF is able to scale well with large networks because it uses areas to break up an autonomous system so that it can minimize all the routing updates that occur during convergence. This is necessary because if you have a network with 50 different routers and there are frequent changes, the SPF algorithm will have to be run every time. And with many routers this can take a lot of time. If you split the 50 router network into 5 different areas the topology tables are only propagated within each area individually. By splitting it up this way the SPF algorithm can run much faster.
OSPF Routers have different roles depending on their location in an area. There are Area Border Routers, Autonomous System Boundary Routers, Internal Routers, and Backbone Routers.
More to come…
This article was meant to be a brief introduction to OSPF and I plan on talking more in depth about different areas of OSPF such as LSA’s, SPF, areas, and configuration.
If you have any specific OSPF questions you would like to have answered please comment below and let me know. I hope your ICND2 and CCNA preparation is coming a long great. Keep up the hard work!
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